Methods of treating cancer by limiting the supply of an amino acid have been suggested previously. For example, asparaginase was found to be the active agent in guinea pig serum that inhibited the growth of certain asparagine-dependent mammalian tumors (see Broome, Nature 191:1114, 1961). Microbial asparaginase isolated from E. coli and E. carotovora were also shown to act as potent anti-leukemic agents (see Howard and Carpenter, J. Biol. Chem. 247:1020, 1972 and Campbell et al., Biochem. 6:721, 1967). When one of these enzymes was utilized in combination with the chemotherapeutic agent vincristine and the corticosteroid prednisone for the treatment of acute lymphoblastic or acute undifferentiated human leukemia, an overall remission rate of 93% was reported (Ortega et al., Cancer Res. 37:535, 1977). Microbial asparaginases resulted in a wide range of host toxicity, however, including hepatic, renal, splenic, and pancreatic dysfunction (Ohno and Hersh, Cancer Res. 30:1605, 1970; see also U.S. Pat. No. 6,251,388).
Another amino acid, arginine, has also been manipulated for cancer treatment. Arginine is considered a semi-essential amino acid because the body can produce some, but usually not all, of the arginine that it requires through a synthetic pathway that utilizes the non-essential amino acids proline and glutamine (or glutamate) as precursors. Newborn mammals are particularly dependent on endogenous production of arginine via the intestinal-renal axis, whereby citrulline is produced in the small intestine and then converted to arginine in the kidneys.
In vitro experiments have shown that a number of cancer cell lines cannot survive more than a few days when arginine is depleted to the micromolar range (Scott et al. Br. J. Cancer 83:800-810, 2000; and for review, Wheatley, Anticancer Drugs 15:825-833, 2004). It is difficult to attain this level of depletion in vivo, however. The prevailing approach has been systemic administration of an arginine-degrading enzyme, which can remove arginine released into the circulation from any one of the potential sources of this amino acid (Tepic and Pyk, U.S. Pat. No. 6,261,557; Clark, U.S. Pat. No. 6,737,259; and Wheatley and Campbell, Pathol. Oncol. Res., 8:18-25, 2002).
In addition to arginases, arginine is also a substrate for nitric oxide synthase (NOS), which converts arginine into citrulline and nitric oxide (NO). There are several types of NOS, reflecting the wide-ranging and very important roles of NO. Arginine is also a substrate for arginine decarboxylase (ADC), which converts arginine to agmatine (and CO2). ADC is present in the brain and kidneys of mammals, but the metabolic role of agmatine remains rather poorly understood. Three additional enzymes that utilize arginine as a substrate are known: arginine kinase, arginine 2-monooxigenase and glycine amidinotransferase.